A hard disk drive storage system typically includes one or more rotating disks, or platters having magnitizible material coated on their surfaces. Read/write heads associated with each platter surface move together radially across the head to reach addressable data regions located on concentric circles called tracks. It is now common to have separate read and write heads. The write head is essentially a small coil of wire which stores data by magnetizing small regions of the disk platter along the tracks. A current driven through the write head creates a temporary magnetic field which magnetizes a small region of the disk at the current position of the write head.
The electronic circuitry used to drive current through the write head typically uses an H-bridge as shown in FIG. 1. For example, U.S. Pat. No. 5,638,012, issued to Hashimoto et al. and incorporated herein by reference, uses an H-bridge for a write driver circuit. The purpose of the H-bridge is to allow electric current to be driven through the write head in either direction. When the current is driven in one direction a magnetic field is created with the north pole in one direction, and when the current is driven in the opposite direction, a magnetic field is created with the north pole in the opposite direction. The H-bridge operates to switch the drive current through the head by turning on a pair of transistors to allow current to flow in a path from a supply source to ground. For example, current flows through the write head from Hx to Hy when transistors Y are turned on and transistors Y are turned off. Similarly, current flows the opposite direction when transistors Y are turned on and transistors Y are turned off. The tr, tf (rise time, fall time) is the time corresponding to the speed at which current can reverse through the inductive load of the HDD write head.
It is desirable to increase the speed of the change of current flow to increase the amount of data that can be stored on a single track of the HDD platter. In order to enhance the preamps write driver performance, the rise time (tr) and fall time (tf) of the write current must decrease without compromising the accuracy of or increasing the transient ringing in the circuit, since increasing the transient ring period on the write current decreases the maximum areal data density. FIG. 2 illustrates the current waveform for a test input to a typical prior HDD write head.
A prior art circuit for reducing the transient ringing is shown in FIG. 3. This circuit was combined with the circuit shown in FIG. 1. The Hx and Hy nodes are connected to the write head in FIG. 1. In this circuit, during transient rings current is dumped to the write head nodes through Zenner diodes. This circuit was somewhat successful in reducing the transients described above but it increased the capacitance at the write head nodes thereby increasing tr and tf.